Because of the limited supply of oil for use by power installations and other high capacity users of oil as a fuel, efforts have been made to substitute or supplement coal as a fuel in place of all or a portion of such oil. One such effort has involved the use of a mixture of coal and oil as a fuel for large users.
In the formation of coal-in-oil mixtures, the oil that is used in combustion systems is supplemented with finely divided coal particles. The coal particles must be of a particle size such that they form a slurry with the oil that is readily pumpable and freely flowable through burners. The coal must also be of a size that the coal particles do not settle or separate out from the oil mixture upon standing.
The pulverizing of coal particles to such a fine particle size also, of course, involves the use of energy. In order to make coal-in-oil mixtures economically feasible, cost of the energy required for such pulverizing must be minimized. Various systems have been proposed for pulverizing coal to the particle size required for stable coal-in-oil mixtures, one of which involves the use of fluid energy mills or jet mills as the pulverizing means. Such mills are well known for use in grinding or drying fine particles, and have now been found to be especially suitable for pulverizing coal.
Fluid energy mills or jet mills use the principal of high speed collision refraction of solid particles in a mill by the use of a charge of heated, pressurized fluid. Generally, a high pressure fluid, such as steam, air or other fluid, is injected through nozzles into a circular device, with the fluid expanding upon passage through the nozzles into the chamber of the mill. The solid particles that are to be pulverized are fed to the chamber and entrained in the fluid stream and swept around the chamber such that they collide with and pulverize each other. The pulverized particles of desired fine particle size are discharged from the device and entrained in spent fluid, while particles that are larger than the desired size are recycled in the chamber for further pulverizing.
The use of steam as the carrier fluid in such fluid energy pulverizing mills, while providing a carrier fluid that prevents combustion or explosion problems within the mill, is expensive annd produces problems associated with cleaning and condensing the steam.